possible to replace 1 of the 2 antennas of a MIMO router with a unidirectional one?

I have to wireless link two apartments, both on the third floor, 700 feet away.
Between the two apartments there are two tall buildings, but between the buildings there is a line of sight from the window of a room in the first apartment to the window of another room in the second apartment.

I have a tight 100$ budget, I want to know if I can use a simple modem-router in the first apartment (like the TP-Link TD-W8960N) and a repeater in the second (like the TP-Link TL-WA830RE), both placed in front of the two windows that face each other.

If it's not enough, or if I wanted to strenghten the signal, can I replace one of the two omnidirectional antennas of the router with a unidirectional 9dbi antenna pointing to the other apartment (like the cheap TP-Link TL-ANT2409A), leaving the other standard omnidiorecional antenna carrying the signal in the apartment itself?
Obviously I'd do the same thing with the repeater, so that the two unidirectional antennas would face each other.
Each antenna will have only a thin glass window and a plastic window shutter in front of it, so it shouldn't be an issue, right?

Is it possible? I don't know if the transmit/receive power of a MIMO router/repeater is split between the two antennas, or if they both work together and have to be replaced only with identical ones, I haven't read anything similar.